This invention relates to a novel, photo-curable resist resin composition, more particularly to a novel resist resin composition based on photo-cationic polymerization containing epoxy resin as the main curable component, and also to a process for preparing a printed circuit board by full additive or partly additive procedure for forming a necessary circuit on an insulating substrate only by electroless plating using the said novel photo-curable resist resin composition, and to a printed circuit board having a resist prepared from the said novel photo-curable resist resin composition.
The cured product of the present resist resin composition has distinguished characteristics against an electroless plating solution, and thus is suitable as a resist for electroless plating to produce a printed circuit board by full additive or partly additive procedure including a step for forming a circuit by electroless plating (both the full additive procedure and the partly additive procedure will be hereinafter referred to as "full additive procedure").
A process for preparing a printed circuit board by a subtractive procedure for forming a circuit on both face and back sides of a substrate and throughhole electroconductive platings by electroless plating, electrolytic plating and etching together has been so far known. On the other hand, the full additive procedure for forming all the circuits and throughhole electroconductive platings only by electroless plating has been recently practically utilized. That is, so many processes for preparing a printed circuit board by the full additive procedure for forming a circuit on an insulating substrate as a starting material only by electroless plating have been proposed, typical of which are the processes proposed by some of the present inventors (Japanese Patent Publication No. 58-30760 and 60-5079), where an adhesive layer is provided on an insulating substrate, the surface of the adhesive layer is roughened or alkali-treated, a noble metal catalyst for electroless plating reaction is provided on the surface, then a resist is provided on other parts than the circuit-forming part, and a circuit is formed on the circuit-forming part by electroless plating.
A process for producing a printed circuit board by the full additive procedure for forming a circuit with an electroless copper plating solution uses a plating solution capable of depositing a plating film having distinguished physical properties such as high elongation and tensile strength. Many such electroless copper plating solutions have been so far proposed. For example, some of the present inventors proposed such an electroless plating solution (Japanese Patent Publication No. 56-27594).
Such electroless copper plating solutions are subjected to plating usually in a strongly alkali state (pH 11.0 to 13.5 at 20.degree. C.) and at a higher temperature of 60.degree. to 80.degree. C. The plating film deposition rate of the electroless copper plating solutions is about 1 to 6 .mu.m/hr, and thus it takes 5-30 hours to form a circuit having a thickness of 30 .mu.m.
Thus, the plating resist for use in the full additive procedure, i.e. a film formed at other parts than the circuit-forming part on the additive layer surface must withstand exposure to the said strongly alkaline electroless plating solution at a high temperature and further must make no dissolution of a portion of the components from the resist, thereby preventing the electroless copper plating solution from contamination and from the consequent decrease in the deposition rate or from the consequent deterioration of the physical properties of plating film. Furthermore, the plating resist is used as a permanent mask, and thus must have sufficient soldering heat resistance, solvent resistance and electrical insulation.
As a plating resist for the full additive procedure that can satisfy these requirements, a screen printing ink of heat-curing type containing epoxy resin as the main component is used, typical of which is that disclosed by some of the present inventors in Japanese Patent Application Kokai (Laid-open) No. 54-13574, or also in the said Japanese Patent Publication Nos. 58-30760 and 60-5079. However, these plating resists require such conditions as 130.degree. to 150.degree. C. for 20 to 40 minutes for curing. Thus, a plating resist of ultraviolet curing type, which can be cured for a few seconds, has been recently in demand.
As resist resin compositions of ultraviolet curing type, solder resists based on radical polymerization are disclosed in Japanese Patent Applications Kokai (Laid-open) Nos. 59-51962, 59-89316, 59-12673, 59-213779, 59-213779, etc., and are also commercially available as a screen printing ink. Furthermore, a resist resin composition for electroless plating based on the radical polymerization is disclosed in Japanese Patent Application Kokai (Laid-open) No. 60-121443. That is, the resist composition disclosed in Japanese Patent Application Kokai (Laid-open) No. 60-121443 comprises an unsaturated compound having at least two double bonds at the terminals as an oligomer such as epoxyacrylate and epoxymethacrylate having acryloyl groups or methacryloyl groups, an unsaturated compound having a double bond at the terminal as a monomer, such as monofunctional or multifunctional acrylate and methacrylate compounds, a photo polymerization initiator, and an organic compound having a thio or dithio group. This plating resist of ultraviolet curing type is characterized by no deposition of platings onto the resist film owing to the presence of the compound having a thio or dithio group as contained. The curing reaction is based on radical polymerization, where the oligomers and monomers undergo polymerization through the radicals produced by the photo polymerization initiator by irradiation of ultraviolet rays.
Furthermore, since photosensitive aromatic onium salts are disclosed as a photo polymerization catalyst for the epoxy resin [Japanese Patent Applications Kokai (Laid-open) Nos. 50-151996, 50-151997 and 50-158680], many photo-sensitive compounds of photo cationic type and resin compositions based on photo cationic polymerization containing epoxy resin as the main curable component and these photo-sensitive compounds as a photo polymerization catalyst have been proposed. In these proposals much diversified uses such as a protective film, an insulating film, a printing ink, a photo resist, etc. are disclosed, but there have been no literatures teaching their use as a resist for electroless plating.
As described above, the resist of heat curing type can be applied by screen printing, but requires much time in curing and also has a low resolving power owing to the running of a printed resist film when heated for curing.
On the other hand, the resist of ultraviolet curing type has a very short curing time and thus has a possibility to solve the problems of the said resist of heat curing type. However, the conventional plating resist of ultraviolet curing type still has many points to improve, such as internal curing property at the curing, and leveling, luster, pinholes, blurring, etc. or printed resist films. Furthermore, the insulation resistance after the plating or moisture resistance test is lower than that of the resist of heat curing type. Still furthermore, uncured monomer components or decomposition components formed by hydrolysis of the ester groups are dissolved from the cured resist film into the plating solution and thus contaminate the plating solution. That is, the conventional plating resist of ultraviolet curing type cannot be used in the production of a printed circuit board according to the full additive procedure.
That is, the present inventors have made extensive studies of the conventional plating resist of ultraviolet curing type based on radical polymerization, and have found that the resist film formed on the insulating substrate by application and successive ultraviolet curing can withstand the highly alkaline electroless copper plating solution at the high temperature only in appearance, but has problems of physical properties of the deposited plating film. Specifically, plating operation is carried out for a duration of 10 hours with an exposed resist area of 200 cm.sup.2 per l of an electroless copper plating solution while supplying consumed components to the plating solution, and this operation is repeated with a fresh resist of the same exposed area but without changing the plating solution, i.e. with the initially charged plating solution. Decrease in the elongation and tensile strength of the deposited plating film occurs at the 3rd and 4th repetitions of plating operation. It has been further found in the heat cycle test of the printed circuit boards obtained after the individual repetitions of the plating operation that the printed circuit boards obtained at the 3rd and 4th repetitions of plating operation with the initially charged plating solution have cracks developed in the throughhole parts with smaller number of heat cycles, as compared with the printed circuit boards obtained at the 1st and 2nd repetitions, and thus have a poor reliability. It seems that unreacted oligomers or monomer components are dissolved into the plating solution from the resist film during the plating operation and accumulated therein, changing the structure of the deposited plating film.
These problems can be solved by replacing the initially charged plating solution with a fresh plating solution entirely after every plating operation or after the 2nd repetitions of the initially charged plating solution, but this procedure is commercially disadvantageous from the viewpoint of mass-production efficiency of printed circuit plates. That is, it is highly desirable to use an electroless copper plating solution repeatedly while supplementing the plating solution with the consumed components.